Optical properties of clad and unclad sapphire fiber

Rick Nubling, Rebecca Kozodoy, and James A. Harrington

Fiber Optic Materials Research Program

Rutgers University, Piscataway, NJ 08855

ABSTRACT

Single-crystal sapphire fibers have been grown, using the laser heated pedestal technique, with losses as low as 0.7 dB/mand with lengths up to 150 cm. We have applied polymer and sol-gel coatings to the fibers and we have evaluated theperformance of these coatings as optical cladding. The best coating seems to be Teflon AF even though the coatings arereasonably thin and low temperature.

1. INTRODUCTION

Solid-core sapphire fibers have many physical properties which make them ideal candidates for infrared transmission upto approximately 3 .5 tm. Sapphire has a melting point in excess of 2000 °C; it has an elastic modulus 7 times greater thansilica; and it is chemically inert. Furthermore, it has an intrinsic (theoretical) optically transparency of 0. 13 dB/m at 2.94 tmand a 340-.tm diameter fiber has been used to deliver more than 600 mJ ofEr:YAG laser energy.' Unfortunately, themeasured losses at 2.94 .tm in sapphire fiber continue to be well above the theoretical limit. Our goal is to fabricate fibers withlosses consistently below 1 dB/m and lengths greater than 1 m. To date, we have grown fibers as long as 1 .5m and, in shorterlengths, with losses as low as 0.7 dB/m at the Er:YAG laser wavelength.

Optical quality single-crystal (SC) sapphire fibers have been studied by several Stanford University groups includingJundt, et al.2 and Feigelson et al.3 They have used the technique oflaser heated pedestal growth (LHPG) to grow 1 to 3 mlengths of 1 10-tm diameter SC fiber. Jundt, et al. report losses as low as 0.88 dB/m at 2.94 m and bending radii of severalcentimeters. Merberg and Harrington and Chang, et al.4 also used LHPG techniques to grow their sapphire fibers. In addition,Saini, et a!. also studied ion-implantation cladding of LHPG sapphire fibers.5 In all cases, the measured losses of LHPG fiberswere found to be higher than the intrinsic loss. These researchers concluded that the source of extrinsic loss was color centerformation during growth, other impurities such as microvoids, and fiber diameter fluctuations. Some improvement in loss wasnoted when the fibers were either grown or annealed in an oxygen atmosphere. Fitzgibbon, et al.6 have used the EFGtechnique to grow SC sapphire fiber. Their fibers tend to have higher loss but the technique of EFG has been used to growmultiple stands of fiber at one time and, therefore, this technique is better adapted to production scaleup than LHPG methods.

2. FABRICATION OF SINGLE-CRYSTAL SAPPHIRE FIBER

There are two common techniques used to grow SCsapphire fiber. One is the EFG method used by Saphikon,Inc.7 and the other is the LHPG method. The mostpopular for growing optical fiber has been the LHPGtechnique, first suggested by Haggerty8 in 1972. In thismethod a CO2 laser is used to create a small molten zoneat the tip of a source rod as shown schematically in Fig. 1.A seed fiber is dipped into the molten sapphire bead andslowly pulled upward forming the SC fiber. Since there isno crucible containing the molten sapphire as there wouldbe in conventional crystal growth, we avoid possiblecontamination from a crucible wall. The LHPG methodhas not only been used by us but also by StanfordUniversity researchers to grow over 30 crystalline fibermaterials. The main components of our LHPG apparatusare the stabilized CO2 laser used as a heat source, the

Fiber

Beltdrlves LaserMike

Focusing minor

CO2 :laser

:( p... .

Reflaxicon optics Turning flat Source

Figure 1 - Diagram of Rutger's LHPG apparatus.

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reflaxicon and associated optics for focusing the CO2 laser beam into a circular ring, a smooth drive mechanism to pull thefiber, and a diameter controller. In addition, the entire apparatus is enclosed in sealed chamber so that the growth atmospheremay be controlled. For most of the fiber growths, we use a power-stabilized (±0.3%) CO2 laser delivering 3 to 25 W of cwpower. In Fig. 1, we indicate how the laser beam is collimated into a ring of light using the reflaxicon and then focused ontothe tip of the source rod using an aspherical reflector. The SC fiber is pulled at a rate of approximately 2 mm/mm and we growfiber with a source rod-to-fiber diameter ratio of 3:1. Our SC source rods are 0.75 to 1.0 mm in diameter and up to one meterin length. For the smallest diameter fibers, we need to regrow fiber in a second pass to achieve fiber diameters in the 100 pmrange. In our work to date, we have grown fibers with diameters from 150 to 350 pm and lengths up to approximately 150 cm.

3.1 Polymer cladding

3. CLADDING SAPPHIRE FIBER

All cladding of the sapphire fibers has been applied after the fiber has been grown. Polymer coatings are in principle easyto apply but they can add additional loss to the fiber because, in general, they absorb in the infrared. We have studied twopolymers, Teflon FEP 100 and Teflon AF 1600. The first is a thermoplastic fluoropolymer which we apply to the sapphireusing a small melt extruder shown in Fig. 2. FEP 100 is a polycrystalline material with a refractive index of 1.3 in the visible

Figure 2 - Screw extruder for applying thermoplastic polymers such as Teflon FEP to sapphire fiber.

region. It has a reasonably high melting point of about 260 °C and, therefore, is good as a cladding on power delivery fibers.The coating rate is 0.5 rn/mm at an extrusion temperature of 380 °C. The cladding thickness varies from 100 to 300 pm.Teflon AF 1600 is a relatively new amorphous polymer which also has a low refractive index of 1.3 and a glass transitiontemperature of 160, °C. This material is applied in the liquid state using a dip coating process as shown in Fig. 3. An on-linefurnace is used to cure the coating in four temperature stages, room temperature, 110, 165, and finally at 330 °C. Since TeflonAF 1600 is only 6% solids in a Fluorinert -75 solvent, the coating applied is much thinner than a melt extruded coating. Ourcoating is about 5 m thick and we have applied up to 5 layers at a rate of 1 rn/mm.

Coated fiber

Nipple filledwith Teflon AFsolution

Figure 3 - Dip coater used to apply Teflon AF to fiber.

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Teflon rod (1/2" dia.)Ball screw

Tungsten carbide pistonHeaters

IGear reducers

H20 cooling

coer' Clutch

'— Drive motor

Fiber

'— Furnace

I Uncoated fiber

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3 .2 Sot-gel cladding

We have also applied alumina sol-gels to the sapphire fiber as a cladding. Alumina gels have the advantage that they arevery high temperature but only thin layer can be deposited in one dip coating. Since the A1203 gel film has a 50% porositywith a mean pore size of 17 Angstroms, the sol-gel will have a lower refractive index than the sapphire fiber and, thus, it willact as a cladding. Our BET nitrogen adsorption/desorption measurements of alumina gel films indicate that the index ofrefraction is about 1.35 for the film in contrast to the value of 1.74 for bulk sapphire at 3 tm.

The alumina gel is synthesized using a Yoldas process9"° standard techniques. Aluminum tn-sec butoxide (ASB) andwater are heated to 80 to 85 °C and nitric acid is added. The solution is condensed to haff of its original volume and allowed tocool to room temperature. This mixture is redispersed in ethanol and the sapphire fiber is dip coated in the so! using atechnique similar to that described above and shown in Fig. 3. Initially, the fiber and cladding are dried at 100 °C;subsequently they are fired at temperatures from 550 to 950 °C for 2 hours in an air or nitrogen atmosphere. The resultingcladding layer is transparent.

4.1 Polymer-coated sapphire fiber

4. ATFENUATION IN COATED AND UNCOATED SAPPHIRE FIBERS

The spectral transmission measured with our FTIR spectrometer for 330-.tm diameter sapphire fiber with and without aFEP coating is given in Fig. 4. The top two curves are for an FEP-coated fiber. This coated fiber is first measured in air andthen immersed in water and remeasured. We see essentially no change in the transmission indicating that the coating ispreventing leakage of the light through the FEP to the water. Yet there is still some residual water absorption at 3 m in thecoated fiber. This is most likely due to water adsorbed on the surface of the fiber and not removed during the post-claddingmelt extrusion process. The other two curves show the uncoated sapphire fiber and again some residual water absorption forthe fiber measured in air. When this unclad fiber is immersed in water then we do see the additional absorption resulting fromthe main water absorption at 3 tm. Clearly, we see that some protective coating is necessary if these sapphire fibers are to beused in a fluid environment with an Er:YAG laser source.

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100

90

80

70Co 60. 5040i-. 30

20

10

05

Figure 4 - Spectral transmission of sapphire fiber with and without a FEP coating. The uncoatedfiber placed in water shows the severity of the water absorption if the fiber is used uncoated at 3

pm.

— FEP /air— - - FEPIH2O

- - - u.c./air

—— u.c./H20

2 2.5 3 3.5 4 4.5

Wavelength, microns

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The sapphire fiber coated with Teflon AF shows results similar to the fiber coated with Teflon FEP. In Fig. 5we see thatthere is little significant difference between the AF coated fiber in air or when it is immersed in water. This means that eventhough the AF coating is considerably thinner (approximately 5 pm) than the FEP coating, it is thick enough to prevent lightleakage into the water. One reason for this is that the index difference between the polymer and sapphire is large and,therefore, there is less evanescent field energy outside the fiber core and a 5 .tm thick cladding is sufficient protection.

100

908070

0. 50

40i- 30

20

10

0

Figure 5 - Spectral transmission of Teflon AF coated sapphire fiber compared to same fiberbefore coating.

We have noticed that therç is sometimes an additional loss in some of the sapphire fibers that we have coated. In Fig. 6we plot the loss at 2.94 p.m for three sapphire fibers with either FEP or AF coatings. We notice the somewhat unexpectedresult that a coated, high-loss fiber has a reduced loss (FEP1) compared to a coated, low-loss fiber (FEP2). This can beexplained by considering the surface roughness of the high-loss fiber. This fiber is not as smooth as the low-loss fiber so whenthe FEP is added the polymer tends to make the core-clad interface less rough and, therefore, there is less loss at this interfaceas there is for the unclad rough surface. The Teflon AF does not alter the loss of the fiber and, thus, this appears to bea goodchoice for a coating material.

— AFIair— - - AFIH2O

• • • u.c.IaIr— — u.c.1H20

2 2.5 3 3.5 4 4.5

Wavelength,microns

5

E

0o Uncoated-J U

Coated

Figure 6 - Loss due to polymer coating on sapphire fiber.

Power In, W

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4.2 Sol-gel-clad sapphire fiber

The sol-gel coated sapphire fiber has presented some difficulties because it has been problem to completely remove waterfrom the coating. In Fig. 7 the spectral transmission is given for a typical alumina-gel coated fiber. The composite coatingthickness is only about 3 tm. We see the water absorption at 3 .tm for this fiber. In this case, we feel that the wateris mainlyin the sol-gel coating rather than on the surface of the fiber.

90

80

70

60C

.5040

! 30I—

20

10

0

Wavelength, microns

Figure 7 - Alumina-gel coated sapphire fiber showing presence of water absorption in thecoating around 3 rim.

5. CONCLUSIONS

We have grown SC sapphire using LHPG methods. The lowest loss at 2.94 .tm has been 0.7 dB/m. These fibers showgreat promise as delivery systems for the emerging Er:YAG laser surgical laser. While we have yet to make meter long lengthsof fiber with consistently low losses, we are improving our growth apparatus and at the same time investigating claddingmaterials. The best cladding material seems to be Teflon AF but it cannot be used at temperatures above about 200 °C. Thesol-gel approach would provide us with a truly high-temperature clad but to date we struggle to remove water from thealumina-gel coatings. We are now trying different drying conditions to reduce the water content of the films.

6. ACKNOWLEDGMENTS

The authors wish to thank Brian Mayer and Chris Jensen for their help in making the loss measurements on the sapphirefibers.

7. REFERENCES

1. G. N. Merberg and J. A. Harrington, "Optical and mechanical properties of single-crystal sapphire optical fibers," AppliedOptics 32, 320 1-3209, 1993.

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2 . D. H. Jundt, M. M. Fejer, and R. L. Byer, "Characterization of single-crystal sapphire fibers for optical power deliverysystems," AppliedPhysics Letters 55, 2170-2172, 1989.

3 . R. S. Feigelson, "Pulling optical fibers," J. Cryst. Growth 79, 669-680, 1986.

4. R. S. F. Chang, S. Sengupta, L. B. Shaw, and N. Djeu, "Fabrication of laser materials by laser-heated pedestal growth,"Proceedings of the SPIE - The International Society for Optical Engineering 410, 125-132, 1991.

5. D. P. 5. Saini, Y. Shimoji, R. S. F. Chang, and N. Djeu, "Cladding of a crystal fiber by high-energy ion implantation,"Optics Letters 16, 1074-1076, 1991.

6. J. J. Fitzgibbon, H. E. Bates, A. Pryshlak, and M. Philbrick, "Sapphire optical fibers for the delivery of Er:YAG laserenergy," Proceedings of the SPIE - The International Society for Optical Engineering, 2131, to be published, 1994.

7. H. E. LaBelle, Jr., "EFG, the invention and application to sapphire growth," J. Cryst. Growth 50, 8-17, 1980.

8. J.S. Haggerty, "Apparatus for forming refractory fibers," US patent no. 4,012,213, March, 1977.

9. B.E. Yoldas, "A transparent porous alumina," Amer. Cer. Soc Bull. 54, 286-288, 1975.

10. B.E. Yoldas, "Alumina sol preparation from alkoxides," Amer. Cer. Soc. Bull. 54, 289-290, 1975.

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